A valve positioner controls the open/closed position of a valve in response to a digital or analog electrical signal. Valve positioners may be used under a variety of conditions. For example, a valve positioner may be exposed to an environment in which a spark from the positioner could ignite explosive fumes. A severe service valve positioner is designed to operate in such conditions without initiating an explosion or other hazard.
One type of valve positioner employs an electro-pneumatic converter to convert an electrical control signal to a pneumatic signal for use in controlling the position of a valve. Often these devices are installed in potentially hazardous explosive environments. To make the devices explosion-proof, the electrical and pneumatic components may be isolated within an explosion-proof housing of the positioner. In the event that maintenance of the valve positioner is required, the electric power is disconnected and, typically, the entire device is removed from the potentially hazardous environment. This is often time-consuming and expensive.
A severe service valve positioner for switching the position of a valve between an xe2x80x9copenxe2x80x9d position and a xe2x80x9cclosedxe2x80x9d position is designed to operate in potentially explosive environments. The positioner includes the following components: a housing structure; a service cartridge, including an electronics compartment and an electro-pneumatic compartment; a pneumatic assembly; and a field termination assembly. The service cartridge is completely modular and detachable from the housing structure, thereby allowing the service cartridge to be removed from the valve positioner quickly and easily for service or routine maintenance.
The electronics compartment includes a controller assembly, which includes a potentiometer having a lockable setting and a reduction gear for locking the potentiometer into place and for increasing the accuracy of a measurement of the angular displacement of the potentiometer with respect to an initial position. The electro-pneumatic compartment includes a current-to-pressure converter known as an I/P motor. The electronics compartment is environmentally segregated from the electro-pneumatic compartment by an I/P cover within the service cartridge. The service cartridge, the pneumatic assembly, and the field termination assembly are physically segregated from each other by the housing structure.
In one implementation of the valve positioner, the reduction gear locks the potentiometer in place by shifting it laterally with respect to a center axis of the service cartridge. In another implementation, the reduction gear locks the potentiometer in place by shifting it axially with respect to a center axis of the service cartridge and engaging detents in the I/P motor base. The axial movement permits environmental sealing of the service cartridge to be maintained by an O-ring located between the hub of the reduction gear and the bore of the hole in the I/P motor base.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.